For the transport of fruit and vegetables a great variety of stackable compact cardboard trays is known, obtained from one or several pieces. Some of these trays are transported to the end user in the form of a stamped sheet of cardboard, with fold lines. The end user mounts the tray in situ, using a special machine and carrying out the gluing of the parts which require to be joined by means of adhesive. Logically, this signifies an additional expense for the end user, who not only requires special machinery but must also undertake the gluing process, etc.
On the other hand, there are the so-called self-mounting and collapsible trays which are glued and folded in origin (since, logically, the empty trays should not be transported erected or mounted, as they would occupy a lot of space) and which are designed for their assembly (self-mounting) in the factory or premises of the end user. That is, these trays are transported to the user already glued and folded, so that in the transport thereof the space occupied is reduced, and the assembly thereof only comprises the erection of the tray. Logically, the trays of this type comprise special means to allow their folding and gluing, by means of a special folder, which means are generally constituted by fold lines, arranged in certain areas of the tray. These trays also usually have means which allow the stabilization of the erect position of the tray, thereby avoiding that the trays, when erected, return to the folded position by the effect of the fold lines and by their tendency to resume the folded position in which they were transported and stored. These means are usually constituted by reinforced corner pillars, of triangular or quadrangular cross-section and which have means of support in the erect position of the tray, which can be flaps or tabs which are lodged in corresponding slots. Examples of said type of tray are described in the patent applications with publication no. GB-A-2205083, EP-A-0394549 and DE-A-3321614.
The stabilization of the corner pillars, in the erect position, can also be achieved by using corner pillars consisting of four sides or segments, two of them fixed to the walls of the box in the area of the corner, with the particularity that the other two sides are narrower than the sides or segments which abut against the walls, in such a way that they allow their folding, about a central folding line, both inwards and outwards with respect to the corresponding corners. When the narrower sides are folded toward the exterior (that is, toward the centre of the tray), they pull the actual walls (that is, the side walls and the end walls) of the box toward the inside thereof, favouring the folding, whilst when they are folded toward the interior (that is, toward the corner), they maintain the box erect, that is, they lock it in its erect position. For this reason, the width of the two narrower sides is slightly greater than the length of the hypotenuse of a hypothetical triangle which would be formed between the corresponding ends of the longer sides abutting on the walls of the box. For the erection of the box, one has to press on the narrower sides of the corner pillar (the imaginary hypotenuse) until suddenly they fold inwards (that is, toward the corresponding corner). Solutions of said type are described in, for example, the patent applications with publication no. NL-A-9000063 and EP-A-0453015.